Mass spectrometry



Oct. 7, B952 G. D. PERKlNs ETAL 2,613,324

MASS SPECTROMETRY Filed NOV. l5, 1949 'l 4 Sheets-Sheet l /04 ,volv 25 46 24 ATTO/UVEK Oct. 7, 1952 G. D. PERKINS ETAL 2,613,324

` 'MASS sPEcTRon/IETRY vf Filed Nov. 15, 1949 4 sheets-Snam 2 //G, 4 /n/avv 4c. caMPa/ve/vr v lV 0 I 277' 477l al w* f fam @iM/k6) Swan/14) @550m/V6 .sz/7760l a Arron/Mer.

OctD 7, 1952 G.' D. PERKINS ET AL, 2,613,324

MASS SPECTROMETRY Filed Nov. l5, 1949 `4 Sheets-Sheet 5 FREQUE/VCY 0' &

/wd im Oct. 7, 1952 G. D. PERKINS x-:T AL 2,613,324

MASS SPECTROMETRY Filed Nov. 15, 1949 4SheetS-Sheet 4 :mfc/Mama; @fer/nf@ Patented -Det. 7, 12952 -l FFICE Mass sPEcTRoMEr'nY 'George Dif. Perkins and Charles F. Robinson, Pasadena,`Calif., assignors to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California f Application 'November 15, 194s, serai No. 127,448

This invention relates to mass spectrometry and to massispe'etrometers.` Morey specifically, the invention is directed to improvements in the method of stabilizing ionbeams and to apparatus for carrying out the improved method.A

A massspectrometer' is an analytical instrument for sorting andqmeasuring ions. Ordinarily, it includes an ionization chamber in which molecules of the sample fto be analyzed are bombarded with' a stream of electrons to convert them 'into ions. rlhey ions formed 4are expelled from' the ionization chamber by one or'more repeller electrodes andare propelled, by so-called propelling or accelerating electrodes, into and through ,anwanalyzer chamberi The analyzer chamberfreiquently takes the form of a curvilinear enclosure, say a' tube. During passage through the chamber, the heterogeneous beam of ions is subjected toa transverse electrical or magnetic eld, or both, to separate it'into separate diverging ionbeams, each of which is composed of ions of'a given mass-tofcharge ratio which differs from the mass-tocharge ratio of the ions forming the otherbeams. Y

The divergin'g beams *may be `successivelyfo cussed through anexit or resolving Aslitin a terminal electrodeontoacollector electrode. Alternatively,"a single "one of the ion vbeams may be continuouslyrio'cussed on the'resolving slit to the exclusion oi'theotherbeams. In either event, the focussing of theion beam or beams is achieved by adjustment oft potential Vapplied to the accele.ratingl electrode The current produc'edby ion'disoharge at the collector electrode is. indicative of theamountof ions inthe particular beam and henceconstitutes a measurer of the partial 'pressure of they molecules (from which the ions were derived) in the vs'amplebeing analyzed;

As indicated above, it may be expedient to focus thel mass spectrometer yon a single masspeak or stepwise on two or more mass-peaks rather than sweepingv the vdiverging ion "beamssuccessively l across the resolvingslit, "I "Such a filxedfo'cuis instrument is used vfor `continuous monitoringv where a continuousconcentration of a single con# stituent in a sample stream is soughuin leak detection where the presence ofy an added contaminant isdetected 'asit appears in the atmoscentered on the resolving slit. Any appreciable deflection of the subject beam from a centered position will aiect the apparent intensity ofthe beam and hence, result in erroneous analysis. Since the factors which govern the position of the beam are in a typical instrument, the trans- Verse magnetic eld and the accelerating voltage, deviation of either of these variables from'a preselected xed value will result in displacement of the beam from a centered position.

It has been proposed to stabilize the accelerating voltagel against deviations from a pre-selected value `by means of a voltage-dropping 'network wherein a fraction of the accelerating voltage is compared with thevoltage output of a standard cell. Any variation vin proportionality between f the fraction and the standard voltage is balanced automatically by a servo-motor or other Ameans actingon the voltage source supplying the ac celeratin'g electrodes. However, the accuracy of focus due to temperature sensitivity, Yvariations in power supply vto an electromagnet, or aging of a permanent magnet is inno way accounted for in such a voltage-dropping'network.

phere surrounding the'jvessel under' ,im/estiga-` tion, and generally underany circumstances in which a single component or pre-selected group of components is-tofbe'measured. v

For accurate Kanalysis with a'xedffocus mass spectrometer, 'it is important thatwtl'ielparticular ion beam under 'investigation 'be' at all times vWerhave developed a method for holding a given ion beam centered on the resolving slit lwhich.automatically compensates for iiuctuations in the accelerating voltage and also variations in the strength of the magnetic eld. The invention contemplates in a method of mass'spectrometr'y.

which comprises ionizing a sample to be analyzed,

`propelling the ions by means of a D. C. accelerating potential into an analyzer region, subjecting the ions in the analyzer region to a transverse Vmagnetic` eld to separate theminto individual ion beams, focussing one of said ion beams through a resolving slit, and' measuring the current developed by discharge of the beam, the improvement comprising superimposing on the D. C.

accelerating potential an A. C. component to cause the ion beam to oscillate transversely with yrespect the resolving slit and with anamplit'ude less than the width of the slit, detectingnuncentered oscillation-of the beam and adjusting `the accelerating voltage to restore centered oscillation.

Thel inventionalso includes one form of appavvratos for carrying out this method, which comprlses in a mass spectrometer having an analyzerA transverse magnetic field in the analyzer chamber, a collector electrode, and a terminal electrode having a resolving slit through which an ion beam is focussed on the collector electrode, the combination comprising a source of A. C. voltage connected to impress an A. C. component on the acceleratingelectrodes tooscillate Athe ion beam transversely withirespect the resolving slit, means for developing an A. C. signal responsive to uncentered oscillation of the ion beam, and means operative to vary the voltage applied from the D. C. source to the accelerating electrodesresponsive to said A. C. signal to restore the beam to a centered position.

The invention will be more clearly' understood by reference to the following details description taken in conjunction with. the accompanying drawing in which:

Fig. 1 is a diagram of a mass spectrometer in accordance with the invention, showing one means of. varying the accelerating voltage responsive to .an A. C. signal developed by an oscillating beam. l .Y Fig. 2 is an enlargeddsometric -view ofthe terminal electrode showingone means of developing yan A. C.- signal',.responsive to oscillation of an ion beam; y y

Fig. 3 isasectionalelevation. takengonrtheline 3-3 of Fig. .2;

Figs. 4, 4A, 4B .arediagram,-.respectivelyf ofthe A. C. signal When-.the meanion beam. position is centered on the yresolving slit, .when :the ion beam is centered to the lett of theresolvingslit, and whenl the. ion `beam is centered to the-.right otthe: resolving slit,- inA each lcase .the amplitude` of beamoscillation beingsuch thatno A. C. signal is developed. Icy-oscillation .ofa centered beam; y l

Figs. 5, 5A .and4 5B are diagramsv illustrating the A. C. signal developedby anoscillatingsion beam,4 the mean position of which ris centered on, to the left of, and totherightof the center ofthe resolving slit, respectively, whentheamplitude of. oscillation is suchfthat an` A. C-. signal is developed by acenteredfbeam;

Fig. -6-V is .-anisometricf'view of the terminal electrodel showing. an alternative means of developing. -an- A. (lrsignalresponsive-to oscillation oi an iorrbeam; and

Fig.=7 is a circuit diagram'of-.the voltagefsupply circuit of Ythe. massispectromet'er of Fig. l showing analternati-ve means'for varying Vthe accelerating voltage responsive to" an A; C.A signal ldeveloped by an oscillating ion beam.

- The mass spectrometer shown diagramriiatically inFig. l includes-apair offacceleratirrg electrodes'IS, II and a repellerfelectr'dde I2. Acg'as sample to be analyzed-is introduced into'their'egion between therepellerfelectrode andthe first accelerating electrode I0 and isabombardedby .an .electronfbeam I3 toionize Athe*molecules Ionsthus-.Yformedare' expelledthrnugh a's'lit IIOA in the rst accelerating electrodeiby application of aupositive potential to .therepeller' electrode. The'expelled ions are 4'accelerated betweenl the electrodes'lil and IIxand .are expelled through slit HAinthe lelectrode IIJVby means .of-apotential Vimpressed'. 'between the two 'accelerating electrodes.

Voltage is supplied to the repeller and' accelerating electrodes from a voltagesupply circuit I4; including a D.` C. voltageV source lconnected in series with a slidewire resistorl I 5. Resistor I6 is tapped by a slider I'I which is connected through a second slide-wire resistor AI8 to the wcircuit for a fixed-focus mass spectrometer wherein any change in the voltage delivered from the source or applied across resistor I8 results inlproportional change in the voltage applied to 4two'/accelerating electrodes and the re Thesi'onssilssuingfrom slit IIA in the electrode II are caused to pursue a curved path as indicated by-the ion lbeam 26, the curved path being produced by a transverse magnetic eld developed by magnet 2'I. A` terminal electrode 30 is placedr in the path of they ion beam after it has travelled/through a arc,A and is provided wltlia .resolving slit'3`I through which a single beam of ion's ofy a given mass-to-charge ratio maybe f'oci'issed.V A collector electrode 32 is disposed behi'njd the terminal -electrode and ions passing' through the' resolving4 slit 3I discharge onlthecollector' electrode'. The collector electrode iselectrically connected t'oa D. C. amplifier 34, which'inturn is connected toa sensing means '3Bwhi'ch` maybe' arecorder" or other sensing device. The curientdeveloped at the collector electrode by discharge of a particular ion beam is amplified in: 'the 'ampliiler 34 and recorded or otherwise sensed in the sensing means 36. this extent, tlieloperation of the mass spectrometer'of Fig. "lis" typical o'i soi-called iixed-focus instruments.

In yaccordance with one form .of the present invention.asillustrate'd in Figs. 1-3, a source 38 o'f A. C'L-volta'ge is coupled through a switch 38A and a transformer 39 to the D. C. voltage supply circuit;- fd to-,superimpose on the D. C. voltage applied to4 the accelerating electrodes a comparatively small A`."C. component of a frequency w whenthe switchacfr i's closed.

A senl's'o'ry velectrode-'42 is mounted adjacent and convnienftlyto the terminalelectrode 30l at one edvftherevlvingslit 3I and is insulated from theterninallelectrode.- A shield is spaced from theffrontof'thevsensor electrode and may be grounded to theterminal electrode 30. The shield is approximatelythe width of the ion'beam so that an-ion beamcenteredonthe resolving slit will not .strikethe sensor electrode y42. The sensor electrodefd'lis connected .to an A. C. ampliiler 45, the .oth'e'r.input. terminale! which is conveniently connected'to gro'undior to terminal electrode 3D (as slio'wnl.

Arever'siblemotor is mechanically connected to slider.. Il .of.the-D..C. voltage supply circuit in s'uch .a.'fasliio1i.that.operation of the motor will .adju'st`tha`-pos'itio`n o'f slider l'Il with respect to the sli'clevvii'e IIL-One coil Aol the motor is connected' toftliyelsource 38 .of.A.v C. voltage and the' ot'h'eilcoil` 4813er' the motoris connected to the output crime-Arc: amplifier 45.

'The' arrangement idf .the terminal electrode, sensor electrodel and shield is best illustrated in the`isometricview' of Fig! 2A and' the sectional elevation cfFig'fS. Referring to Athese guresthe electrcdell is pr'ovdeclvvith` anA elongated rectangular resolvingl sli`t13'l'. Sensor electrode 42 is mounted;across" one end' of the slit and convenientlyidirectly' toelectrode .30 by means of in- 'j sulating spacers 48, 49. A shield 44, conveniently comprising a conductive metal. strip,I is rigidly affixed at 44A to the electrode 30 and offset vtherefrom to project across and above the central part of sensor electrode 42. The end of theshield overlying the sensor electrode is approximately the same width as the ion beam plus thepeakco-peak amplitude of beam oscillation, and is in Vertical alignment with the resolving slit, such that the part of the beam which strikesshield 44 is centered on the shield when the portion of the beam which passes through slit 3| is centered in the slit. It may or may not result that shield 44 is centered above slit 3l since the ion beam may be more or less curved. The terminal electrode 30 and the sensor electrode42 are connected by leads 30A and 42A, respectively, to amplifier 45 (shown in Fig. l). Alternatively,` the terminal electrode and amplifier may be grounded. f

The operation of the apparatus illustrated in Figs. 1-3 is best described with relation vto the diagrams of Figs. 4, 4A, 4B. The voltage output of the A. C. source is held at such a valuethat the resultant oscillation of the ion beam has an amplitude just less than the width of the shield. Hence, if the beam is perfectly centered.r on the resolving slit, oscillation thereof vwill'be within the region defined by the shield 44 (see Fig. 2), and the beam will not fall upon the sensor electrode 42 in any part of the oscillating cycle.. This condition is illustrated in Fig. 4 wherein the base line 52 extends through two complete cycles of the A. C. component having a frequencyw. e

Beneath the base line in the figure, the relationship of the ion beam to theexit slit 3| and shield 44 is illustrated through various stagesin the oscillating cycle. Thus at zero amplitude, the

beam is centered on the resolving slit and shield.

through the other half of the cycle,shifting-to ward the right of the shield and again back to the zero or centered position tocomplete the cycle. Since the amplitude of oscillation ofthe beam is less than the width of the shield, it does not strike the sensor electrode and no signal .is

produced.

In Fig. 4A, the same type of chart illustrates the conditions existing when the beam is out of focus to the left of the center of the resolving slit. At zero amplitude, the beam is adjacent the left edge of the resolving slitbut still on the shield, and a quarter of a cycle later it has shifted to the left, falling outside the resolvingslitand shield. In this position, the tail end of the beam strikes the sensor electrode and a pip 54 is developed across leads 36A and 42A and delivered to the A. C. ampliiier 45. A quarter of a cycle later the beam shifts back to its zero position, thereafter shifting to the right of center` of the resolving slit and at the completion of a cycle, back to the zero amplitude position. adjacent the lefthand edge of the shield. In this manner a signal is developed between the sensor and terminal electrodes for each cycle of oscillation, the pips 54, 54A on base line 52 indicating the `relationship of these signals to the beam position.

The same kind of lanalysis is carried out in Fig. 4B showing the signal developed when the mean beam position or zero amplitude position is to the right of center of the resolving slit and not shown) .6 y shield. Under these ,conditions asingle pulse is developed during each cycle of oscillation, illustrated as the .pips 56, 56A on the base line 52. However, thesignal developed when the beam is tothe right of center is opposite in phase to the signal developed when the beamte the left of centen although both -signals have a fundamental frequency of. w Where the frequency of.y the voltage output ofthe A. C; source is a. Underthese conditions, the outputA of the A. C. amplifier impressed yacross motorv coil 46B (Fig. l), will be of the same frequency as the signal impressed across motorcoil 48A from the A. C. source 38. 'I'he motor will `thusbe energized in the direction, depending upon thephaseof the output of the A. C. amplifier, Vto `adjust the slider IT in the appropriate directionv to return the beam to a centered position; Y f The foregoing discussion describes vthe casein which the shield 44 has width equal to the beam Width plus the peak-to-.peakemodulation amplitude. A second situation which Vmay -arise either accidentally or deliberatelyin practice is one in which the shieldv width4 is of the same order as the beam width, being less than the beam width plus the peak-tomcak amplitude. In this condition, the results are as'A shown in Figs. 5, 5A, 5B, which show the A. C. signal developed on electrode 42 responsive to the three conditions of beam position, li.- e., centered, de-

lcentered. left and decentered,y right -.respectively.

In ,each of these situations-,fa signal of lfrequency 2o and higher is developedon electrode 42,-but only in the decenteredpcases 5A, 5B Adoes this signal contain a component of frequency w which is required yto drive the motor 45. It is thus noted that although the superficial; appearance of the signal depends strongly on; the relative widths of the beam, the shield, and the modulation amplitude, the actual `operation of the inventiony is insensitive to changes in these parameters within wide limits.

It is readily apparent that .the function of shield 44 is simply toprevent current of frequency w from being collected on sensing electrode 42 when the beam is properly centered, and this function -rnay be served equally well by simply cutting away part yor allof the central portion of electrode 42, as-shown in Fig. 6, which is an .isometric view similar to Fig. V2.

In the modification shown in Figpf, a senso;- electrode 60 is divided centrally into segments 60A, 60B-' The electrode segments are mounted on terminal electrode 3D with their adjoining edges spaced on opposite sides of the longitudinal axis of resolving slit .3L The sensor electrode segments are insulated from the terminal electrode by insulators 6l', 62, respectively, andare connected by a single lead 64'fto the amplifier The intervening space between the segments performs theA same-function as shield 44 (Fig. 2).

An important advantage of the invention lies in the fact that under no condition is it necessary to permit the central partof the ion beam to be impaired in any way.` Accordingly, the practice of the invention permits' detection and correction of a decentered beam long before the degree of decentering is large enough that the beam strikes an edge ofv slit 3l rand impairs accuracy of the analysis by so doing.

The method Aand apparatus of i the' present invention thus has the fundamental and very important advantage that the D. C. output of the collector electrode 32 is uninterrupted and meer at the collector electrode isfnot affected'so' long r as the ion beam iscentered on the'slit. NIf either of the `critical variables, to-wit, vaccelerating voltage or magnetic iield -strength, vary from 'a pre-selected value', the beam focus lwill'shift in one direction or another from center .position ..1

and will immediately develop a signal at'th'e sensor electrode of the type shown in Figs. 4A or 4B, 5A or 5B. The -reaction of the centering circuit will be substantially instantaneous to effect `a compensatory' adjustment *0f 'the-fac'- 'l is also used as the reference peak, such a stepper celerating lvoltage 'to-:return the beam :to a centered position. yForuthis reason, any I error introduced into the Acollector electrode discharge signal, as a consequence of variationinbeam focus, will be of insignificant duration.l

the foregoing description, a :servo-motor has been illustrated and described asv one means of adjusting the acceleratingv voltage responsive to production of a-n A. C.- signal ofthe .samerfreryThe invention has been further described with respect toffits operation orina: single 4mass peak wherein the mass-Speak' of interest s 'also used a's'the basis "for centeringltheapparatus.. kThis is 'not'anecessary limitation ofy the apparatus or they method since itmay be frequently desirable to center on a relatively stronger peak v`even thoughf such. a relatively stronger peak is of. no interestin the analysis.v This is simply accomplished by placing astepper switch in the D. C. supply circuit I4, which may be manually or -autoinaticallvoperated periodically to shift the accelerating voltage to that required .to focus 'the stronger peak 'on `the resolving slit. .Such a stepper switch ini the D; C. supply circuit is con,- venientlyganged to 'the switchSSA in the A. C. voltage {supply circuit so that an A. C. compo.- nent isimpressed on theD. C. supply circuit only when thereference peak is focussed on the resolving slit. Of course, if the peak of interest switch is not necessary in the D. C. supply circuitandthe A. C. component may be impressed on--the D. C-.lvoltage continuously.

Application ofthe invention isnot limited to the'particular apparatus illustrated as preferred quency as the beam modulation voltage; "Another means of accomplishing this adjustment is yshown in the circuitdagram of Fig. '7,whch includes a voltage supply circuit substantially similar to the circuit I4 of ."Fig. 1. .The circuit68 in.-

cludes a D. Cfvoltagefsource E9 anda voltage divider network 'I0 connected to the accelerating electrodes (-notshown). Anoscillator 12 is connected through a', transformer 1.3 to apply an A. C. voltage Ato the J circuit. A `synchronous rectifier 14 is connectedto receive'the signal from the A. C. amplifierfnot shown) and a syn'- chronizing signal fromioscillatorlz. Whenwthe input signal has the sameffrequencyas the synchronizing signal, therectifier.v Lwhich is-aconnected to circuit l68 across a resistor'l, develops i a D. C. voltage across .the'f'resistorfthe .polarity of which is sensitive to the phasero'f'the input signal. Hence, the voltage across thedivider network 10 is varied responsive to the signalxdeveloped by anon 'centered oscillatingbeam and E.

in a direction depending vnon .the 'phase of the signal.

The invention has been described in some detail with relation to 'a mass spectrometer operating on only apre-'selected mass peak. Such an L tion distinguishes from-a continuous scan mass4 spectrometer-in which the accelerating voltage is varied gradually;-over,a g;iven range to sweep a part or all o ftliegmass spectraof -a Ygiven sample successively across the resolving slit.

When the accelerating voltage is varied step-V wise in a continuousfmonitor instrument .to monitor two .or morecornponents, the beam centering methodand apparatus .of the .invention is useful to holdxall .of .thesuccessivfe beams in a centered position. v

means for accomplishing the improved operation. It is entirely practical to produce'the desired oscillation of the ionv beam by means other vthan impression .of an A. C. voltagecomponent on the acceleratingelectrodes.` For example, auxiliary electrodes 'arranged transversely with respect to 'the p'athof ion iaf'z'ivelwill induce the same .type o'ffbs'clllationwvhen energized Hwith an A. CL voltage. Alternatively, the magnetic field may be varied in a yrhythmic manner toaccon1plishthe same purpose. At the same time, the correction applied b y motor 46 (Fig. 1), by thesynchro'nous rectier 'M (Fig. 7) or by other'means sensitive 'to the Fourier component of the sensorelectrode output of the frequency of the -beam oscillation, is not restricted to' the -adjustment of the accelerating potentials. Variation in the magnetic fieldst'rength or an auxiliaryeld developed by deflecting electrodes will also accomplish the des ired adjustment of the beam position with respect to the exit slit.

1. In mass spectrometry involving ionizing a sample to'be analyzed, propelling the ions by means of a D. C. accelerating potential into an vanalyzer-region, subjectingthe ions inthe analyzer region to a transverse magnetic ield to separatethem into individual ion beams, focussing one-of 4said ion beams through a resolving slit, discharging the beam passing through the .resolving slit, and sensing the current developed by'disch'arge of the beam, the improvement comprisingsuperimposing on the D. C. accelerating .potential an A. LC. component to cause the ion beam to oscillate transversely With respect to the resolving slit and with .an amplitude less than that which would cause thebeam to strike aside edge of lthe slit, detecting uncentered oscillation of the beam and adjusting the accelerating voltage to restore centered oscillation.

fsingonecbfr-said 4ion beams through the resolving sli-t,}'a-nc l sens'ing'the `current developed by dis- -eharg'eof' the beam, the' improvement comprising periodically superimposing on the D. C. accelerat- 'ing potential an A. C. component to causethe ion-beam to oscillate transverselyY with respect slit, discharging the beamV passing through the resolving slit, and sensing the current` developed by discharge of the beamfthe improvement-comprising superimposingy on the D. C. accelerating potential an A. C. component to cause theion beam to oscillate transversely with respectto the resolving slit and with an amplitude less vthan ythat which would cause the beam to strike a side edge of the slit, detecting uncentered oscillation of the beam and adjusting `the accelerating. po-

tential responsive tothe detected uncentered oscillation vof the beam .to restore centered oscillation. r

`Il. In mass spectrometry involving ionizing a sample to be analyzed, propelling the ions by means oi a D. C. accelerating potential into an analyzer region, subjecting the ions in the analyzer region toa .transverse magnetic iieldto separate them into individual ion beams, focussingone of said ion beamsthrough a resolving slit in-a .terminal electrode, discharging the beam passing through the resolvingslit, and sensing the current ydeveloped by discharge of the beam,

the improvement comprising periodically superimposingon the D. C. Vaccelerating potential an A. C. component to cause the ion beam to osi cillate transversely with respect to the resolving slit yand with an amplitude lless than that which would cause the beam to strike a side edge of the slit, whereby the oscillating beam generates an A. C. signal when it is not centered on the resolving slit, and adjustingthe accelerating potential responsive to such A.`C. signal to restore centered oscillation. a y

5. In mass spectrometry involving ionizing a sample to be analyzed, propelling the ions by means of a D. C. accelerating potential into an analyzer region, subjecting the ions in the .analyzer region to a transverse magnetic eld to separate them into individual ion beams.' focussing one of said ion beams through a resolving slit, discharging they beam passing through the resolving slit, and sensing the current developed by discharge of the beam, the improvement comprising continuously superimposing on the D. C. accelerating potential an A. C. component to cause the ion beam to oscillate transversely with respect to the resolving slit and with an amplitude less than that which would cause the beam to strike a side edge of the slit, whereby the oscillating beam generates an A. C. signal when it is not centered on the resolving slit, and adjusting the accelerating potential responsive to such A. C. signal to restore centered oscillation.

6. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, .a source of D. C. voltage connected to impress an accelerating potential on the accelerating electrodes, means for establishing a transverse magnetic field in the analyzer chamber, a

collector electrode, and a terminal electrode having a resolving slit through which an lon beam is focussed on the collector electrode, the combination comprising a source of A. C. voltage connected to impress an A. C. componenten the accelerating electrodes to cause oscillation of the ion beam, means independent of said collector electrode, .to detect uncentered oscillation lof said beam and to develop an A C. signal-responsive to such uncentered oscillation, and means operative to vary the voltage applied from the D. C. sourcev kto the accelerating electrodes responsive to said A. C. signal. f

7. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, a source of D. C. voltage connected to impress an accelerating potential on the accelerating electrodes means for establishing a transverse magnetic field in the analyzer chamber, a collector electrode, and a terminal electrode having a resolving slit through which an ion beam is focussed on the collector electrode, the combination comprising a source of A. C.-voltage connected to impress an A. C. component on the accelerating electrodes to oscillate the ion beam transversely with respect to the resolving slit, means independent of said collector vfor developing an A. C. signal responsive to uncentered oscillation of the beam, and vmeans operative torvary thevoltage applied to theaccelerating electrodes responsive to said A. C. signal to restore the beam to la centered position. Y

8. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodesa source of D. C. voltage connected to impress an accelerating potential on the accelerating electrodes, means for establishing a transverse magnetic eldin the analyzer chamber, a collector electrode, and a terminal electrode having a resolving slit through which an ion beam is focussedion the -collectorwelectrode the cornbination comprising a source of A. C.vvoltage connected to impress an A. C.c omponent on the acceleratingl electrodes to causeV oscillation ofthe ion beam, the source lof A. C. voltage being adapted to produce an A. C. component at'the accelerating electrodes of such magnitudeas yto cause beam oscillation of anamplitude just less thanV the Width ,of` said resolving slit,l means cooperating with said terminal electrode and independent of said collector electrode to detect uncentered oscillation of said beam and tomde- `velop an A. C. signal responsive to such uncentered oscillation, means operative to vary the voltage applied from the D. C. source to the accelerating electrodes y'responsive to said A. C.k

9. In a mass spectrometer having'an analyzer chamber, ionization chamber, accelerating electrodes, a source of D. C. voltage connected to'impress an accelerating potential on the accelerating electrodes, means for establishing a trans'- verse magneticfeld in .the analyzer chamber, a collector electrode, and al terminal electrode having a' resolving slit through which an ion beam is focussed on the'collector electrode, the combination comprisingasource of A. C. voltage connected to impress an A. C. componenten the accelerating electrodesf'a sensor electrode ldisposed adjacent thet'erminal electrode on the side opposite the collector electrode, [an A. C., amplier connected'to the sensor electrode, means operativeto vary the voltage applied from the D. C. source to the accelerating"electrodes'jresponsive to theoutput of said A1C. amplifier." f

10. In a mass spectrometer havingran analyzer "chamber, ionization chamber, accelerating electrs.' a source 'offD .nC'. voltage connected to impress' an'accel'erati-ng-L potential on the accelerating" electrodes; means for establishing av transyer'se magnetic field inthe analyzer chamber, a

collector electrode,'and a terminal electrode'hav- -i'n'g"a" resolving slit through which anion beam isiocussed on the collector electrode, the combination Icomprising a source of A. C. voltage connected to impress an A. C. component on the accelerating electrodes, a sensor electrode disposed adjacent one end of said resolving slit in Vthe focussing electroc'ie on the side opposite the press an accelerating potential on the accelerat-` ing electrodes, -'means Vfor `establishing a transverse xnagnetic'elcl in the `analyzer chamber, a collector electrode.- and a termina/l electrode having 'a resolving "sli-tthrough which an ion beam is -focussed onthe collector electrode, the combination 'comprising a sourcenof- A. C. voltage connected to impress-anl Al C. component on the'- accelerating electrodes,r a sensor electrode disposed adjacent one end of. said resolving slit inthe v'term'ir'ial electrode on 'the side opposite the collector electrode and insulated from' the terminal electrode, an' 'electrical shield mounted iii-'front of the sensorelectrodejand 'grounded to 'the terminal electrode,y anA. C. amplifler'connectedto the sensor electrode, and mea-ns oper- Vatiye rto vary the voltage applied from'the; D. C. sourceI to the accelerating electrodes responsive -tothe outputrof said A. -C. amplifier;

"12. Apparatus according to Vclaim 1-1'- wherein the 'sensory Aelectrode is 'mounted jto the terminal electrodealjacent oneA vend of fthe-resolving slit and is spaced-"from the terminalelectrode insulat'in'g spacers.-

- V1"3. Apparatusa'ccordng to claim 12A wherein 'the electrical shield isY affixed 'td'the "terminal electrode in conductive relationship and-is oiset outwardly to `overlie the 'central portionof the sensor 'electrode `without malfxir'igl electricalv connection thereto.

14.. Apparatus according to jclairn 11l wherein the means operative to vary thev voltage f'cornprises a two-phase reversible motormechanically 'connected to 'saidv source fof "BL C voltage.' one coil of the motor 'being Jcon'nected Vto the source oi' A; C. voltagefand fthe other "coil ofthe 'motor v'being v(zonneste'dto fthe A. C. ampliilen Apparatusaccording uto* claim 11'1 'wherein the means operative "to vary the voltage vv'cornprises 'a synchronous rectifier conne'ctedftcjrelc eive "the "output signal from sai'd KA; Cramplin'er ais aninput'signal Yand a'signai 'fromsaid A. C. "source as 'a synchronizing signal 'and' tol develop trodes, a source of D. C. voltage .connected to impressk an accelerating potential! onf Vaccelerating electrodes, means for 'establising a transverse magnetic eld in the analyzer chamto the output ma.; such therme ,ampli e y 1-2 bena collectorelectrode; and a terminal elec.a trodehaving a resolving .slitthrough which an ion kbeam is focussedI onftheicollector electrode, the vcombination comprisingwa source of A. C. voltage-connected `to impress, an A. C. component on the acceleratingelectrodes, asensor electrode disposed adjacent oneV end, of;y said resolving slit in theV focussing electrode .onthe side opposite the collector electrode and insulatedfrom the terminal electrode,l the sensorl electrode being non-receptive to iondischargein theregion overlying an extension oi?k rthe, longitudinal; axis` @f1 @118.18-

solving slit.

17.` Apparatus. according .in claim., .16; wherein the.. sensori electrodercomprlses twoy physically separated-electrically connectedsegments, spaced from each otheronf opposite sides ofthe longitudinal i axis. of. the4 resolving; slit, f

v318.81114?. mass spectrnmeierhaying ananalyzcr chamber,- 'ionization chamber. accelerating electrodes, a `sourcesof D. cfyoltageconnectcd to impress :an accelerating potential: onthe accelerating electrones;` means for establishing a transverse magnetic -iield in the analyzer chamber; a terminali` electrodehayinga resolving, slit, a' sensor. electrode disposed .infront ofv Vthe terminal electrode transversely with respect to. ion slit, andmeans preventing the ion discharge in an 'area' defl'xeozll :by said. sensor electrodeand overlyingf'the ."longi'tudinalaaid's of` thev slit .and narrower-than the slit, 'the'.method of holding an ion 'beam in'focus oni the center of said slit ccmprising' impressi'xigfantkf iC. .potential fon said accelerating electrodes toloscillate the ion beam, maintaining. the'magnitude of .saidAz C. potential such thatvfthe A'amplitude of beam oscillation plus the average`width1ioif' the beamxis rgreater than the transversewidthiot said area and v'less than the transcerse fwidth ofsaidxiesolving slit.

1.9.1n'a mass' spectrn-ieterhavl ari-analyzer chamber, ioni'zationcfhajmber, accelheifatifn'g relectrodes ,a 'source ofi-l? voltage cenne'ctedto impress an accelerating 'cen-'tialj on the accelerating electrodesrn sfofi' e tablish'i-ng'fa transvers/e magnetifi f1; inthe .analvzechambn a terminal cleared? having itsQlriig- .'Slit *a sensorelectrode'disposed' in front of "-the terminel elstrodeiranverselywith respect if@ Said ionfslit. and meapsnrventins thei'n .discharge in fan area' *A denedjliy said, sensor electrode and @suriname 'lnsitudlhallaxis of ne. su@ ,and

narfowerfthanthesl .Entier 0f Seiden? C0111' nrisinaimpfe ,sl fpintl 0,11, Seid accelerating lele" 1trodensj to ,'olscillatejtlie'uion beam, maintaining thejrnagnit e 'of said A, "Cxpotenl l n e eofjbeamsoscillation plus the average,widthJ ofjtlvie jbeam is jless than the 'transverseiwidtn efr-sam area,

20.111 mass `'sp ectrometry 'invpiving ionizing ya sample4 to jbeanalyzed, Qsuljjecting the .ions to a combination, of. electric', and.magnsliijl fields 5to separate vthe fion ',bev'arnliinto a "p luretlitgyv .of "divergentbeams ea',clrha'ying affcharacteris mass- 'tocharge."ratio,` andgpassing one suche, divergent beam'thrcughiaresclrmasmimm iaolliector. the improvement-)wbr "comprises causing one Aof said beamsV Ltooscil te transversely 4with respect to the. resolvingslit, discharging', Ia part ofjthe oscillating,seguimiento itsfreachmg the izeso1v1ngs1it, and; adjusting thejpostion of me `beam withirsspgetjtq.theslijcresponsite ,to the Fourier componentof-the .dischargesignal the frequency of which is 'the same as 'the 'frequency at which the beam is oscillated.

13 21. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes for propelling ions from the ionization p chamber, means for establishing a transverse magnetic vfield across the analyzer chamber, a

collector electrode, and a terminal electrode be tween the analyzer chamber and collector elec# -trode and having a resolving slit therein, the

combination comprising means for oscillating an ion beam transversely with respect to said resolving slit, an auxiliary collector electrode being insensitive to a beam passing centrally through the resolving slit, anoll means operative to adjust the position of the beam responsive to the Fourier component of the auxiliary electrode output which is of the same frequency as the frequency of oscillation of the ion beam.

GEORGE D. PERKINS. CHARLES F. ROBINSON.

No references cited. 

